This is a great question, and one that everyone has when they decide to tackle a touch up paint project using aerosol spray cans. It can be a little confusing to estimate how many cans of primer, paint or clear coat you need. Obviously, the size of the area you are going to spray determines how much you need. But it goes further than that, because each coating (primer, base color, clear coat) may need more or less than the other, depending on number of coats you are spraying, and if you are spraying the whole panel, or just part of it (e.g., see primer below).
Below are some rough rules of thumb for each paint, along with a table that estimates the total number of cans needed assuming typical panel sizes and typical coverage amounts. Of course, your mileage may vary, so you need to adjust these estimates up or down based on your particular case.
This coat typical needs the least amount of paint, because often, you don’t need to prime the entire panel. You are spot-priming, which means you need enough to cover the bare metal or plastic only. This is the area of the panel that has been sanded down to the bare metal or plastic. Check your directions on the number of coats you are going to apply (at least two), and estimate accordingly.
The base coat always requires multiple wet coats, either of the whole body panel, or of a smaller region that you have masked off. For example, if your repair is below the waistline of the door, you might just spray the lower half of the door, up to the trim. This would reduce your need for paint. However, if the whole lower part of the door was primed due to extensive chips that were sanded to the bare metal, you might need an extra coat or two of base coat to cover the primer. This might increase your need for paint back to the original estimate for doing the whole door panel. Take your particular repair into consideration when estimating the amount of paint you need.
You are always going to clear coat the entire area that you painted, and you might apply 3-4 coats of clear coat to achieve extra gloss and extra chip protection (especially on a hood or front bumper). Therefore, your clear coat estimate may be on the higher side.